Dihydroxylation of Alkenes to give 1,2-diols (vicinal diols)
Description: Treatment of alkenes with osmium tetroxide (OsO4) leads to the formation of 1,2-diols (“vicinal diols”).
Notes: The reaction proceeds with “syn” stereochemistry on the alkene, meaning that the two alcohols end up on the same side of the alkene. Sometimes a reducing agent like NaHSO3 or KHSO3 is added to remove the osmium at the end. Regardless, the key reaction is the same.
Also note that cold, alkaline KMnO4 does exactly the same reaction.
Mechanism: Osmium tetroxide adds to one face of the alkene through a cycloaddition reaction (Step 1, arrows A, B, and C) to give a cyclic osmium compound (“osmate ester”). The osmate ester is then reduced with NaHSO3 or KHSO3 and hydrolyzed to the diol through a very long process that is, to be frank, excruciatingly tedious to write out and not generally bothered with in Org 1 and Org 2.
Notes: There is also a variation where OsO4 is used in catalytic amounts and a stoichiometric oxidant such as N-methylmorpholine N-oxide (NMO) is used to regenerate OsO4.